Exemplary embodiments herein generally relate to a valve operating mechanism for an internal combustion engine, and, more particularly, to a deactivating hydraulic valve lash adjuster/compensator with temporary lash compensation/deactivation for improved switching response.
As is well known, valve lash is the mechanical clearance in a valve train between a camshaft and a valve in an internal combustion engine. Valve lash is usually about 0.2 mm to 0.3 mm depending on the engine specifications. Valve lash is intended to provide the greatest amount of valve opening on the high point of a camshaft lobe and assure that the valve is tightly closed on the low segment of the camshaft lobe. Hydraulic valve lash adjusters (HVLA's) or hydraulic valve lifters are widely used to eliminate service required to compensate for valve wear. The HVLA's use engine oil pressure to establish a continuous zero valve lash dimension under all conditions in the vehicle engine. While the valve is closed, the internal piston of the hydraulic lifter is lightly thrust against the pushrod by engine oil pressure to eliminate all valve train clearance. When the camshaft high spot comes around, the hydraulic lifter's fill hole is covered and the lifter acts like a solid piece of metal, and the valve opens. Thus, the HVLA ensures that the valve train always operates with zero clearance, leading to quieter operation and eliminating the need for periodic adjustment of valve clearance.
To improve fuel economy, cylinder deactivation is also widely used. Cylinder deactivation is the deactivation of the intake and/or exhaust valves of a cylinder or cylinders during at least a portion of the combustion process. In effect, cylinder deactivation reduces the number of engine cylinders within which the combustion process is taking place. With fewer cylinders performing combustion, fuel efficiency is increased and the amount of pollutants emitted from the engine will be reduced. For example, when such a system is installed in a six-cylinder engine during cylinder deactivation the valves are shut off and fuel supply in two cylinders or three cylinders is halted depending on the driving conditions. Deactivating the cylinders means that, when cylinder deactivation is in operation, it's the same as driving a car with a smaller, lower-displacement engine, emitting less CO2. Cylinder deactivation deactivates the cylinder(s) by keeping the intake and exhaust valves in the closed position to halt fuel supply. To deactivate the cylinder, one example includes a pin that is moved hydraulically to disengage the rocker arm that pushes down the valve. In this mode, even though the cam pushes up on the rocker arm, it has no effect and the valve remain closed. Cylinder deactivation is effective, for example, during part-load conditions when full engine power is not required for smooth and efficient engine operation.
When valves with HVLA's are deactivated, the adjuster can eliminate the lash that is required to re-engage the valve mechanism. This is sometimes described as “pump up”. When this occurs, once the valve drive mechanism is reactivated, the valve lash is too small, so that even when the cam is on the base circle and the valve should be closed, the valve can remain open. When a valve is open that should be closed, combustion gasses leak, power drops, and the valve could quickly overheat and fail, destroying the engine. Pump up can be mitigated by choking the oil supply to the HVLA. However, this also reduces the overall switching response by removing a portion of the working energy.